In recent years, MEMS gyroscopes have been adopted rapidly in a variety of consumer applications due to significant reduction in their size, cost and power consumption. Conventional vibratory rate gyroscopes generally use a pair of low-frequency rigid-body resonance modes in a microstructure for rotation rate detection, by actuating the primary resonance mode of the device and detecting the rate-proportional Coriolis displacement signal along the secondary resonance mode. Although these vibratory gyroscopes provide the degree of functionality required by some consumer applications, they can fail to offer the performance level demanded by many high-end applications, such as short-range inertial navigation, while maintaining a micro-scale physical size.
By taking advantage of the stiff bulk resonance modes of the device structure, high-frequency resonant bulk acoustic wave (BAW) gyroscopes can overcome many limitations of low-frequency gyroscopes, such as vibration sensitivity, susceptibility to mechanical shock, and inadequate bandwidth and dynamic range under mode-matched conditions. In some cases the mechanical rate sensitivity of gyroscopes can decrease at higher resonance frequencies due to the smaller vibration amplitude and the distribution of mass and stiffness of bulk resonance modes over the volume of the device. High-sensitivity capacitive BAW gyroscopes generally utilize submicron air gaps and large DC voltages to provide efficient transduction at high frequencies. They can also require vacuum encapsulation to avoid squeeze-film damping, which in turn necessitates special design considerations for co-integration of capacitive gyroscopes with static accelerometers, where low-pressure requirements for gyroscope packaging conflicts with the desired over-damped performance of the accelerometer needed for fast settling time and small overshoot.
The quest for implementation of BAW gyroscopes that can provide efficient in-air transduction to minimize packaging complexity of multi-degree-of-freedom sensors, without the need for narrow gaps and large DC polarization voltages to further reduce fabrication cost and high voltage requirements of the sensor, has led to the implementation of piezoelectrically-transduced high-frequency resonant gyroscopes. Inherent linearity and high efficiency of the piezoelectric transduction combined with superior power handling of thick single-crystal silicon acoustic platform can facilitate actuation of the piezo-on-silicon gyroscopes with adequate vibration amplitudes, paving the way towards significant enhancement of rotation rate sensitivity and total signal-to-noise ratio. Although the piezoelectric thin film can provide effective transduction and thus large drive amplitude, an efficient frequency tuning mechanism is needed to enable mode matching of all-piezoelectric high-frequency resonant gyroscopes in the presence of process non-idealities. This can be accomplished with a multi-port BAW gyroscope, utilizing a gyroscopic mode pair, with mode matching capability, enabled by a dynamic frequency tuning technique based on electrical feedback of the drive-mode displacement signal.
Further, silicon BAW resonant disk micro-gyroscopes operating in mode-matched condition at high frequencies have recently been considered as a viable miniaturized solution for rotation-rate sensing. Capacitive signal transduction through nano air-gaps has also been considered for BAW silicon disk gyroscopes.